The general objective of the proposed research is an increased understanding of the processes by which children and adults perceptually and conceptually differentiate categories of objects into more specific subcategories throughout the continuum of expertise. Experts are superior to novices in terms of the number of categories recognized at subordinate (e.g., sparrow, jeep) and sub-subordinate (e.g., chipping sparrow, Cherokee jeep) levels. Consequently, experts are able to discriminate among domain-specific exemplars with considerably more precision than novices. Little is known about the effects of developmental level or level of domain-specific knowledge on individuals' acquisition and extension of sub-basic categories, or about the mechanisms that drive subordinate categorization throughout the continuum of expertise. To address these issues, two experiments are proposed. The first experiment will involve an examination of differences among children and adults who are novices, intermediates and experts on dinosaurs in both the selection of features on which subordinate categories are based, and in the inferred functions of visible physical features of subordinate categories. Subjects will be tested on categories from both the familiar (dinosaur) domain and an unfamiliar (shorebird) domain in order to determine whether particular aspects of categorical knowledge are domain-specific or domain-general. The second experiment will provide the first in-depth longitudinal analysis of transitional phases involved in the acquisition of expertise on a new domain by children and adults. Quantitative and qualitative differences in both sub-basic categorical knowledge and categorical inferences will be examined, and potential changes in the location of the basic level will be tracked. Both experiments will involve comparisons across children and adults with comparable levels of domain-specific knowledge, and among same-age individuals with different levels of domain- specific knowledge, in order to evaluate the effects of both strategy use and domain-related knowledge on subcategorization. The results of these studies will provide a basis for developing a theoretical model of the mechanisms involved in the acquisition of expertise on categorical domains. Importantly, the results also will provide guidelines for intervention programs designed to facilitate lexical and cognitive development. Understanding the processes involved in expertise acquisition should reveal factors that may facilitate or accelerate the acquisition of cognitive competence, which is an important determiner of self-esteem. Since level of self-esteem is a critical index of mental health, helping both normally and atypically developing individuals to acquire expertise would be an important step toward improving and maintaining mental health.